I've done a bit of research on the topic. We're going to do one for the Fall Foliage Classic.
This is a copy of the proposed idea for my group. I think something similar would work well with the shorter 70 mile run Howard found.
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1. We arrive at our predetermined start location
-Drivers are given a map with directions to the next point.
-Cars are sent out at XX minute intervals.
2. Cars arrive at Checkpoint A where they receive a poker chip.
-Drivers (or passenger) are asked to compete a task (hop on 1 foot for 60 seconds for example)
-When task is completed Driver (or passenger) receives 2nd card, map and directions to next point.
3. repeat step 2
4. repeat step 2
5. All drivers reach point 4 and do the last task.
We take off as a group and finish the remaining tour en masse.
I like the idea because it gives us all a chance to drive individually and to cruise as a large group.
6. When we arrive at our final destination hands are made with the 5 cards each car should have. The best hand wins a prize.
The poker chip itself shows that each player made it to the checkpoint, cards are easy to fake/cheat and adds a keepsake/memento for the event.